Curing of preceramic articles with gaseous hydrogen halide

ABSTRACT

A novel curing process for preceramic organosilicon polymers is disclosed which involves contact with a gaseous hydrogen halide.

RIGHTS OF THE GOVERNMENT

This invention was made with Government support under a subcontract withDow Corning under Prime Contract No. F33615-83-C-5006 awarded by theDepartment of Defense (DOD). The Government has certain rights in thisinvention.

FIELD OF THE INVENTION

This invention relates to the production of preceramic andceramic-shaped articles from organosilicon polymers.

BACKGROUND OF THE INVENTION

Ceramic materials are of critical importance for a number of hightemperature, high performance applications such as gas turbines. Theseapplications require a unique combination of properties such as highspecific strength, high temperature mechanical property retention, lowthermal and electrical conductivity, hardness and wear resistance, andchemical inertness. Design reliability and the need for economicalfabrication of complex shapes, however, have prevented ceramic materialsfrom fulfilling their potential in these critical high temperature, highperformance applications.

The design reliability problems with ceramics, and the resultant failureunder stress, are due largely to the relatively brittle nature ofceramics. This, in combination with the high cost of fabricating complexshapes, has limited the usage of ceramics.

Ceramics made from organosilicon polymers have the potential to overcomethese problems. To this end, polymers based on silicon, carbon and/ornitrogen and oxygen have been developed See, e.g., "Siloxanes, Silanesand Silazanes in the Preparation of Ceramics and Glasses" by Wills, etal., and "Special Heat-Resisting Materials from Organometallic Polymers"by Yajima, in Ceramic Bulletin, Vol. 62, No. 8, pp. 893-915 (1983), andthe references cited therein.

The major and most critical application for ceramics based on polymerprocessing is high strength, high modulus, shaped articles such asfibers. Such fibers are spun from organosilicon preceramic polymers, andthen cured and pyrolyzed to their ceramic form. The low molecular weightand highly branched structure of typical preceramic polymers, however,alters the spinning and subsequent fiber handling behavior of thesepolymers from that of conventional polymers. In particular, gelation andfoaming tendencies in the melted polymers used for melt spinning maylead to the presence of undesirable flaws in the resulting fiber. Suchflaws are undesirable in fine diameter fibers since they are believed tobe the source of cracking and lowered tensile strength. Furthermore,because of the low molecular weight of the preceramic polymers used, thefibers spun therefrom have relatively low tensile strength and aredifficult to handle in spinning, curing, and subsequent pyrolysisoperations.

One important step in the formation of shaped articles such as fibersinvolves curing the preceramic polymer prior to pyrolyzing the same.Although various curing techniques, such as oxidative/hydrolytic cures,are known in the art, nevertheless, a need exists for developingimproved curing procedures which will render the fibers inert tomorphological changes other than the desired densification as thefilaments are pyrolyzed to ceramics.

Typical curing procedures of the prior art are described in U.S. Pat.Nos. 3,853,567, 4,535,007, and 4,399,232, wherein various curing agentsare described.

The instant invention involves the use of a hydrogen halide such ashydrogen chloride, hydrogen fluoride, hydrogen bromide, hydrogen iodideor mixtures thereof as a curing agent in order to obtain improvedresults including rapid curability, increased green strength (strengthbefore pyrolysis) as well as increased strength of the shaped ceramicarticles. The most preferred curing agent is hydrogen chloride.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 represents a preferred method of carrying out the novel processof this invention.

In said FIG. 1, a preceramic polymer is melt spun using, for example, aram type extruder. A polymer rod (2) is pressed onto a hot grid surfaceand the resulting molten polymer is forced through the spin pack and outthe spinnerette (3). The fiber is drawn by controlling the take-up speedof the take-up reel (8) and the throughput of the polymer throughspinnerette (3). The fiber is passed through a curing chamber (4) whichcontains the curing gas (5) which is introduced evenly around the fiberby means of a loop of tubing (6) with holes (7) facing toward the fiber.The entire system is protected from oxygen and moisture by means of anisolation chamber (1) flushed with an inert gas such as nitrogen.

SUMMARY OF THE INVENTION

Accordingly, a primary object of the present invention is to provideimproved processes for the production of preceramic and ceramic fibersfrom organosilicon preceramic polymers involving the use of a novelcuring agent.

Another object of the present invention is to provide improved meltspinning and dry spinning processes for the production of fine diameter,organosilicon preceramic fibers and ceramic fibers having high tensilestrength, made therefrom.

Another object of the present invention is to provide improved processesfor the production of preceramic and ceramic fibers based uponorganosilicon preceramic polymers, which fibers have improvedhandleability, e.g., increased toughness and protection of theorganosilicon preceramic material from abrasion and the atmosphere.

Still another object of the present invention is to provide improvedprocesses for the production of improved preceramic fibers based uponorganosilicon preceramic polymers, in which the organosilicon polymermaterial is protected from degradation by the oxygen and moisture inair.

These and other objects, aspects and advantages, as well as the scope,nature and utility of the present invention, will be apparent from thefollowing description and appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The Organosilicon Preceramic Polymers

Organosilicon preceramic polymers are well known in the art. Suchpolymers contain silicon, carbon and/or nitrogen, and are fiber-forming,and can be cured and pyrolyzed to ceramic form. See, e.g., U.S. Pat.Nos. 4,310,651; 4,312,970; 4,342,712; 4,482,689; and 4,340,619; whichare incorporated herein by reference.

These organosilicon precursor polymers may be made in a variety of waysas is known in the art. For example, they may be made by firstdechlorinating an alkylchlorosilane, e.g., dimethyldichlorosilane, andpolymerizing the product to form a polysilane, e.g., polydimethylsilane.This material is then heated to convert its backbone of silicon atoms toa backbone of alternating silicon and carbon atoms by forming apolycarbosilane.

Preferably, the organosilicon preceramic polymers utilized in thepresent invention consist essentially of silicon, carbon, Hydrogen,nitrogen and oxygen. Such polymers are typically prepared by reacting adisilazane and a dichlorodisilane or methylchlorodisilane.

Most preferably, the organosilicon preceramic polymers of the presentinvention are characterized as polysilazanes prepared fromtrichlorosilanes and hexamethyldisilazane. Particularly preferred arethe polysilazanes, containing N--Si--N linkages. Optionally, theaddition of difunctional monosilanes as co-reactants may be used toenhance spinning and/or subsequent fiber handling properties. Suchdifunctional monosilanes include preferably R₁ R₂ SiCl₂, where R₁ and R₂may independently be a hydrogen, methyl, ethyl, phenyl or vinyl group.

Such organosilicon preceramic polymers may be further modified, forexample, by incorporating vinyl functionality by reacting with thepolymer itself. This may be achieved, for example, by co-reacting thepolymer with a vinyl (Vi) halosilane such ViR₁ R₂ SiCl, where R₁ and R₂may each independently be methyl or phenyl.

Another preferred type of organosilicon polymer which is thermallysensitive and which may be especially suitable in the present inventioncomprises a plurality of cyclic and/or linear precursor residues of therepeating units of formula I: ##STR1## linked together by Si₂ W₂ bridgesof formula II, ##STR2## wherein R is hydrogen, a lower alkyl grouphaving from 1 to about 6 carbon atoms, a substituted or unsubstitutedvinyl group, a substituted or unsubstituted alkyl group, a substitutedor unsubstituted benzyl group, a substituted or unsubstituted lower arylgroup having from 6 to about 10 carbon atoms, a tri(lower)alkyl ordi(lower)alkylsilyl group, a di(lower)alkylamino group, a lower alkoxygroup having from 1 to about 6 carbon atoms and n is an integer greaterthan one. The substituted groups are substituted with lower alkyl andlower aryl groups.

These polymers form novel ladder-like or planar array structures thatare soluble in common organic solvents, stable at room temperature andthermally stable up to about 200° C. The ladder-like or planar arraypolymers of the present invention are formed in high yield by reactionof the cyclic and/or linear residues in the presence of a basic catalystcapable of deprotonating an NH function adjacent to silicon.

These polymers and their preparation are described more fully in U.S.Pat. No. 4,482,669, to Seyferth et al., assigned on its face toMassachusetts Institute of Technology, which patent is hereinincorporated by reference.

Molecular weight (M_(n)) for the above organosilicon preceramic polymersmay vary from about 300 to 20,000, preferably about 700 to 5,000, andmost preferably 1,000 to 2,000 (g/mole-GPC method). These polymers mayalso have softening temperatures (T_(s)) of about 40° C. to 308° C.,preferably about 60° C. to 200° C., and most preferably 70° C. to 150°C.

Spinning of the Fibers

As indicated earlier, the above-described organosilicon preceramicpolymers are dry spun, melt spun or extruded as fibers or filaments.

To melt spin, the solid organosilicon polymer is melted at a temperatureand rate sufficient to avoid gelation and foaming, and substantiallyimmediately thereafter the melted polymer is preferably spun or extrudedto form optically flaw-free, fine diameter organosilicon preceramicfiber.

Prior to spinning, any gel detected in the polymer blend should beremoved such as by filtration. In addition, the polymer should beessentially free of other contaminants such as small insolubleparticulates or bubbles.

The relatively short residence time of the polymer in the melt iscritical to achieving optically flaw-free, fine diameter fibers. If anorganosilicon polymer is brought up to a melt-processable or extrudabletemperature and held at such temperatures for too long a period time,gelation will occur, which in turn will lead to the presence ofnoticeable flaws in the fiber and a concomitant loss of tensileproperties. In addition, the melt temperature of the organosiliconpolymer should be less than that needed to cause foaming of the polymer,which foaming will also lead to the presence of voids or flaws in thefiber. The typical organosilicon polymer is significantly more meltsensitive as compared to other conventional fiber-forming polymers,e.g., polyethylene terephthalate.

The actual melt temperatures may vary, but will generally be above thesoftening temperature (T_(s)) of the organosilicon polymer, but belowthat at which foaming, gelation or other degradation occurs within thetotal melt residence time. Typically, such melt temperature will rangefrom about 30° C. to 130° C., and most preferably 60° C. to 80° C. abovethe T_(s) of the polymer blend.

As the preceramic fibers are melt spun or extruded, fiber handling ispreferably minimized to avoid abrasion of the fibers sufficient to causefiber breakage during fiber take-up and/or sufficient to induce latentstressing sufficient to cause fiber breakage during subsequent curingand pyrolysis to ceramics. Thus, those type of conventional fibertake-up apparatus which induce high levels of stress in fiber arepreferably not used. The preceramic fibers as spun are relativelybrittle due to their relatively low molecular weight as compared toconventional fiber-forming polymers.

The preceramic fibers as spun may be taken up in any appropriate take-upspeed. Take-up speed of up to about 1400 meters/minute, typically 300 to1000, and preferably 500 to 800, may be used.

To dry spin, the solid organosilicon polymer is dissolved in a solventat a relatively high polymer solids concentration, and thereafter thepolymer is spun or extruded to form flaw-free organosilicon preceramicfiber.

Any solvent in which the organosilicon polymer may be dissolved at therelatively high solids concentration may be used. Suitable aliphatichydrocarbon solvents may include those having from 1 to 8 carbon atomsand having boiling points ranging from about 0° C. to about 190° C.Typical aliphatic hydrocarbon solvents include n-hexane, cyclo-hexane,cyclo-hexene, n-pentane, cyclopentadiene, iso-octane, acetonitrile,dichloroethane, trichloroethane, hexachloroethane, chloroform,methylchloroform, methylene chloride, methyl acetate, ethyl acetate,carbon tetrachloride, and tetrahydrofuran. Suitable aromatic hydrocarbonsolvents may include those having from 6 to 10 carbon atoms and haveboiling points ranging from about 70° C. to 180° C. Typical aromatichydrocarbon solvents include toluene, xylene, styrene, benzene,chlorobenzene, dichlorobenzene, ethylbenzene, and isopropylbenzene.Toluene and xylene are particularly preferred.

Prior to spinning, any gel detected in the polymer should be removedsuch as by filtration. In addition, the polymer should be essentiallyfree of other contaminants such as small insoluble particulates.

As indicated above, the relatively high solids concentration of thepolymer in the spinning solution is critical to achieving aself-supporting threadline with these low-molecular weight polymers. Ifinsufficiently high and organosilicon polymer solids concentrations areused, threadline breakdown will frequently occur. Preferably, polymersolids concentrations of at least about 70 percent, and most preferablyat least about 95 percent are used.

After dissolution and prior to spinning, the polymer should bemaintained in solution in an essentially gel-free state, i.e., keptunder conditions insufficient to cause gel formation of polymer in thesolvent.

The actual solution temperatures at spinning may vary, but willgenerally be near the boiling point of the solvent (to improve solventevaporation) but below that at which foaming, gelation or otherdegradation occurs within the total dry spinning residence time.Typically, at spinning such solution temperatures will be between about70° C. and 250° C., preferably 70° C. to 200° C. and most preferably 90°C. to 160° C.

As the preceramic fibers are dry spun and solvent-extracted, fiberhandling is preferably minimized to avoid abrasion of the fiberssufficient to cause fiber breakage during fiber take-up and/orsufficient to induce latent stressing sufficient to cause fiber breakageduring subsequent curing and pyrolysis. Thus, those types ofconventional fiber take-up apparatus which induce high levels of stressin fiber are preferably not used. The preceramic fibers during and afterspinning and solvent extraction are relatively brittle due to theirrelatively low molecular weight as compared to conventionalfiber-forming polymers.

Curing of the Spun Fibers

The novel process of this invention involves curing the preceramicfibers as spun by contacting the same with gaseous hydrogen halide whichis preferably admixed with an inert gas such as nitrogen and argon, etc.The curing of the preceramic polymer can take place during either of twostages. it can be cured in a bath manner after it has been formed into adesired shape, e.g. fibers can be made and cured after they have beenplaced on a take-up reel. The most preferred method of cure, however, isto treat the fiber immediately after it is spun or before it is placedon the conventional take-up reel. The preferred curing technique can beconveniently carried out using the procedure shown in FIG. 1.

The amount of hydrogen halide (e.g., hydrogen chloride) utilized is notnarrowly critical and it can be introduced at any suitable concentrationranging from 100% to a 0.05 mol percent mixture of the same with aninert gas. The residence time is also quite flexible and can range from0.0001 to 30 minutes depending on such factors as the concentration ofthe hydrogen halide, the thickness of the shaped polymeric article, etc.

The temperature employed for curing can range from room temperature upto the glass transition temperature of the particular polymer. It ispreferred to operate at temperature at least 20° C. below said glasstransition temperature.

Following the above-described curing, the preceramic polymers aresubject to pyrolysis conditions which render the fibers ceramic.Typically pyrolysis conducted in inert atmosphere of nitrogen argon orthe like, pyrolysis temperatures may be from about 600° to 2000° C.,preferably 850° to 1800° C. and most preferably 1100° to 1400° C.

EXAMPLE 1

An organosilicon preceramic polymer is prepared according to the generalprocedure of U.S. Pat. No. 4,535,007 and is introduced into a meltextruder after filtration. The polymer is passed through a spinneretteat a temperature of 180° C. to produce fibers which are collected on atake-up reel.

Lengths of fiber were removed from said take-up reel and placed in aceramic container which was then placed into a curing chamber whoseenvironment and temperature could be controlled.

Gaseous hydrogen chloride (100%) was introduced into the curing chamberat about 150 cc/min. for about 15 seconds while the temperature wasmaintained at about 40° C.

The fibers were then subjected to the following heat treatment cycle inan argon atmosphere.

    ______________________________________    Temperature           Time    ______________________________________     40° C.        1     hr.    40-300° C.     1     hr.    300° C.        0.5   hr.    Cool to room temperature (RT)    RT-500° C. .sup.                          1     hr.    500-1200° C.   2     hrs.    Cool to RT    ______________________________________

The above procedure resulted in the production of ceramic fibers.

EXAMPLE 2

The procedure of Example 1 was repeated using trichlorosilane in placeof the hydrogen chloride.

The trichlorosilane was introduced as a 0.08 mole % mixture with argonand the curing time was 5 minutes.

EXAMPLE 3

The procedure of Example 1 was repeated with the exception that 100%boron trichloride (BCL₃) was used instead of hydrogen chloride and thecuring time was 2 minutes.

The ceramic fibers of Examples 1-3 had the physical properties shown inthe following Table 1.

                  TABLE 1    ______________________________________            Tensile Strength                         Elastic Modulus                                     Fiber Diameter    Example Ksi.sup.1    Msi.sup.2   (Micrometers)    ______________________________________    1       159          22.6        11.3    2       80           19.4        12.7    3       38           16.6        11.6    ______________________________________     .sup.1 1,000,000 lbs/sq.     .sup.2 1,000 lbs/sq.

As can be seen, the hydrogen chloride cure (Example 1) resulted inceramics which had superior properties than like products made withprior art curing agents (Examples 2 and 3)

EXAMPLE 4

An organosilicon preceramic polymer is prepared according to the generalprocedure of U.S. Pat. No. 4,535,007 and is introduced into a meltextruder after filtration. The polymer is passed through a spinneretteat a temperature of about 180° C. and immediately passed through thecuring chamber of FIG. 1 into which a gas mixture of hydrogen chlorideand nitrogen is continuously flowed at about 150 ml/min. and is thencollected on a take-up reel.

Fibers were removed from said take-up reel to yield pieces having alength of about 18 inches.

The 18-inch fibers were suspended in a furnace and subjected to thefollowing heat treatment profile in a nitrogen atmosphere.

    ______________________________________    Temperature           Time    ______________________________________    100° C.        1     hr.    Cool to RT    RT-500° C. .sup.                          1     hr.    500-1100° C.   2     hrs.    Cool to RT    ______________________________________

Eight separate batches of fibers from said heat treatment profile werethen evaluated for physical characteristics and the results are shown inthe following Table 2.

                  TABLE 2    ______________________________________           Tensile Strength                        Elastic Modulus                                    Fiber Diameter    Batch  Ksi.sup.1    Msi.sup.2   (Micrometers)    ______________________________________    1      189          22          14.6    2      204          23          13.3    3      215          24          14.0    4      204          23          13.0    5      138          23          20.9    6      192          23          14.7    7      177          22          16.8    8      172          21          18.0    ______________________________________     .sup.1 1,000,000 lbs/sq.     .sup.2 1,000 lbs/sq.

EXAMPLE 5

The procedure of Example 1 was repeated with the exception that thepolymer used was prepared according to the procedure of U.S. Pat. No.4,482,669. The curing time was 1 minute instead of the 15 seconds ofExample 1--all other details being the same as said Example 1.

The physical properties of the ceramic fiber were as follows:

    ______________________________________    Tensile Strength                  Elastic Modulus                              Fiber Diameter    ______________________________________    30 Ksi        9 Msi       50 Micrometers    ______________________________________

In addition to the improved tensile strength and elastic modulus, theresulting ceramic fibers have a diminished carbon content as a result oftreatment with hydrogen halide for reasons which are not completelyunderstood. As is known in the art, free carbon is not deisrable inceramic fibers of the instant type and thus the curing agents of thisinvention have an added advantage.

What is claimed is:
 1. A process for producing a ceramic material whichcomprises:(a) contacting a shaped article made from an organosiliconpreceramic polymer with a gaseous hydrogen halide for a period of timesufficient to cure the same; and (b) thereafter pyrolyzing said shapedpreceramic article by heating the same in a controlled atmosphere atelevated temperatures.
 2. The process of claim 1 wherein said shapedarticle is a fiber.
 3. The process of claim 2 wherein said contactingwith hydrogen halide is done immediately after the fiber is spun.
 4. Theprocess of claim 1 wherein said hydrogen halide is hydrogen chloride. 5.The process of claim 1 wherein said hydrogen halide is hydrogenfluoride.
 6. The process of claim 1 wherein said hydrogen halide ishydrogen bromide.
 7. The process of claim 1 wherein said hydrogen halideis hydrogen iodide.